The heart has a natural pacemaker and conduction system which causes the heart muscle to contract, or beat, in an orderly rhythmical manner. The normal pacing rate for an adult at rest is about 60 to 70 beats per minute. There are many physiologic abnormalities which cause one or more chambers of the heart to beat more rapidly (tachycardia or flutter) or chaotically (fibrillation). A patient cannot live with ventricular fibrillation because there would be no blood pumped through the arteries, but may live with atrial fibrillation so long as the chaotic impulses are filtered out at the AV node and do not reach the ventricals. A patient may also live with atrial flutter and various forms of tachycardia but quality of life may be considerably compromised.
Many of these arrhythmias can be treated effectively by ablation using radio-frequency (RF) energy. Other arrhythmias are less effectively treated, requiring more RF lesions for a successful outcome or resulting in no successful outcome. RF ablation is performed with a catheter having one or more electrodes which deliver the RF energy to the cardiac tissue. In operation the catheter is guided through a vein or artery into the heart chamber and positioned at one or more sites, determined by an electrophysiologist, to correct the arrhythmia. The catheter delivers energy from an external source (generator) to the tissue, generating sufficient heat to kill the tissue, which is thereafter replaced by scar tissue. In a successful ablation procedure, the lesions formed interrupt the electrical pathways that cause the arrhythmia so that heart rhythm is improved or returns to normal.
During ablation, it is important to control the temperature of the tissue, both at the tissue surface and below. If the surface temperature becomes excessive, dehydration and charring. results. Charred tissue presents a dangerous situation as it may flake off resulting in blockage of a blood vessel. Excessive heating below the surface is also dangerous as it may result in a "steam pop." A "steam pop" occurs when deep tissue is heated to a temperature sufficient to boil the water of the tissue which creates a steam bubble within the tissue. Such a steam bubble erupts through the surface of the myocardium with substantial force. This eruption can typically be heard as a "pop" by the electrocardiologist.